The present invention relates to a ball screw device, and more particularly to improvements of a structure for fixing a ball-circulating part embedded in a ball nut.
As types of circulating balls in a ball screw device, the following two types are conventionally known. Namely, one type is arranged such that, as shown in FIGS. 1 and 2, by using a ball-circulating tube 4 disposed in a ball nut 2 threadedly engaged with a screw shaft 1 via a multiplicity of balls 3, the balls are circulated by being introduced into the ball-circulating tube 4 from ball screw grooves of the screw shaft 1 and the ball nut 2. Another type is arranged such that, as shown in FIG. 3, by using a ball-circulating piece 5 disposed inside the ball nut 2, balls 3 are introduced into a groove formed in the ball-circulating piece 5 and are circulated by riding over a ridge between adjacent ball groove portions of the screw shaft 1.
In these circulating-type ball screw devices, an adhesive is used to fix the ball-circulating tube or the ball-circulating piece. The adhesive is filled in a space 6 which is formed after the ball-circulating tube or the ball-circulating piece is fitted in the ball nut. In addition, as the adhesive, a material is used whose mechanical properties, such as coefficients of thermal expansion and elastic displacement, are very similar to those of the ball nut member, and which undergoes a very small change in volume when it is hardened is used.
However, with the above-described conventional ball screw devices, the process for sufficiently cleaning and degreasing the surfaces of the ball nut 2 and the ball-circulating tube 4 or the ball-circulating piece 5, which serve as surfaces to be bonded, becomes necessary in order to maintain the bonding strength of the adhesive. In addition, the time required until the adhesive becomes hardened and stable after assembly is long. Further, after the adhesive has become hardened, the operation of removing a portion which has risen from the outer peripheral surface of the ball nut is required. Thus, there is a problem in that the assembly of the ball screw device is very time-consuming and troublesome.